As known per se, an electrolyzer of water vapor (H2O) at high temperature, or HTSE (high-temperature steam electrolysis) electrolyzer, comprises a stack of a plurality of elementary solid oxide electrochemical cells. Referring to FIG. 1, a solid oxide cell or “SOC” 10 particularly comprises:    a) a first porous conductive electrode 12, or “cathode”, intended to be supplied with water vapor for the production of hydrogen,    b) a second porous conductive electrode 14 or “anode”, through which the oxygen (O2) generated by the electrolysis of the water injected onto the cathode escapes, and    c) a solid oxide (dense electrolyte) membrane 16 sandwiched between cathode 12 and anode 14, membrane 16 being an anion conductor for high temperatures, usually temperatures higher than 600° C.
Heating cell 10 at least up to this temperature and injecting an electric current/between cathode 12 and anode 14 causes a reduction of the water on cathode 12, which generates hydrogen (H2) at the level of cathode 12 and oxygen at the level of anode 14.
A stack 20 of such cells, aiming at producing a significant quantity of hydrogen, is illustrated in the simplified view of FIG. 2. In particular, cells 10 are stacked on one another while being separated by interconnection plates 18. Such plates have the double function of ensuring the electric continuity between the different electrodes of cells 10, thus allowing the electrical serializing thereof, and of distributing the different gases necessary for the cell operation, as well as, possibly, a carrier gas for helping the draining off of the products of the electrolysis. To achieve this, plates 18 are connected to a water vapor supply 22 for the injection of this vapor onto the cathodes of cells 10 in accordance with a constant water vapor flow rate DH20, determined by a controllable valve 24. Plates 18 are also connected to a gas collector 26 for the collection of the gases originating from the electrolysis. An example of a stack and of an interconnection plate structure is for example described in document WO 2011/110676.
Such an electrolyzer may also operate in co-electrolysis, that is, with a gas mixture at the cathode input formed of water vapor (H2O) and of carbon dioxide (CO2). The mixture at the anode output is then formed of hydrogen (H2), of water vapor (H2O), of carbon monoxide (CO), and of carbon dioxide (CO2).
For the effective implementation of the electrolysis by stack 20, the stack is taken to a temperature greater than 600° C., usually a temperature in the range from 650° C. to 900° C., the gas supply is started at a constant flow rate and an electric power source 28 is connected between two terminals 30, 32 of stack 20 to have a current/flow therethrough.
The tightness between solid oxide cells 10 and interconnection plates 18 is usually achieved by seals which are one of the weak points of the system. Such seals ensuring the tightness of stack 20 towards the atmosphere of the hot area are fragile and may allow the leaking of:                hydrogen and water vapor if the leak is located on the cathode side, and/or        oxygen if the leak is located on the anode side.        
However, due to the high temperature of the electrolyzer, there exists around it an area having a temperature close to the temperature of the stack, and thus an area capable of reaching temperatures higher than 650° C. Now, such temperatures are higher than the self-ignition temperature of hydrogen (571° C.). The area around the electrolyzer where the temperatures are sufficient for the self-ignition of fuels is usually called “hot area”. It generally corresponds to the thermally-insulated enclosure containing the electrochemical device. With no specific security measure, there thus is a risk of fire, or even of explosion, if the leaks cause an accumulation of hydrogen close to the electrolyzer. To guarantee the security regarding the risk of explosion, the hot area is usually swept with an air flow sufficient to achieve the combustion of any fuel gas leak and avoid the accumulation of hydrogen. Particularly, the enclosure having the electrolyzer housed therein comprises an inlet having air injected therethrough, and an air outlet, which thus enables to circulate the air in the enclosure and thus regularly renew its content. Thus, fuel gas is never accumulated and there is no risk of explosion. However, such a solution imposes a very high flow rate of air, which has to be preheated to the enclosure temperature to avoid cooling the electrolyzer, this being particularly disadvantageous in terms of power efficiency.
Even though such measures avoid the risk of explosion, they do not enable per se to detect a leak of the electrolyzer, and accordingly, they do not enable to warn the user of the electrolyzer or to implement an automatic disabling of the electrolyzer. Several leak detection systems have been developed for this purpose.
A first solution is based on the exothermicity of the reaction of combustion of the hydrogen having leaked from the electrolyzer, which reaction provides a flame at more than 2,000° C., which causes a rise in the temperature of the hot area. In practice, one (or a plurality of) temperature sensor(s) is (are) thus arranged in the enclosure housing the electrolyzer to measure the temperature in the hot area. An electronic package may thus be connected to the temperature sensor(s) and automatically turn off the electrolyzer when the measured temperature exceeds a predetermine detection threshold. Such a solution however has a low accuracy. Indeed, a same temperature increase of a thermocouple in the hot area may be due to the radiation of the hydrogen flame of a small leak close to the thermocouple or of a strong leak distant from the thermocouple. To avoid an erroneous leak detection, the detection threshold is thus oversized, which thus amounts to only detecting strong hydrogen leaks.
A second solution comprises placing in the enclosure a hydrogen detector, or a hydrogen explosimeter, to measure the hydrogen content of the gas sweeping the hot area, and thus detect a hydrogen leak. However, hydrogen sensors do not operate beyond a given temperature, and particularly the temperatures of a hot area of an electrolyzer. The hydrogen sensor is accordingly placed in a cooler area, placed downstream of the hot area in terms of air flow. An analysis of the signal supplied by the hydrogen detector can then be performed to determine whether the measurement exceeds a threshold requiring placing the electrolysis system in safe conditions. However, the combustion of hydrogen essentially occurs in the hot area, so that the sensor can only detect a minute portion, or even none, of the hydrogen if the leak is small. Indeed, only a strong hydrogen leak, resulting in the total consumption of the oxygen, induces a detectable presence of hydrogen at the sensor level. Only a strong hydrogen leak can thus be detected.
Sensors capable of operating directly in the hot area have been developed. However, they do not provide a real advantage since they only detect the hydrogen which has not burnt in the sweeping air, which again corresponds to the case of a strong leak.
Further, the developed solutions aim at a direct or indirect detection of a hydrogen leak. They thus do not enable to detect an oxygen leak on the anode side of the electrolyzer.
In other words, there exists no solution in the state of the art enabling to detect a leakage both on the cathode side and on the anode side of a HTSE electrolyzer, and which is capable of detecting small leaks.
A high-temperature solid oxide fuel cell, better known as a SOFC, has similar problems. Indeed, a HTSE electrolyzer and a SOFC are identical structures, only their operating modes being different. Referring to FIG. 3, an electrochemical cell forming a SOFC comprises the same elements (anode 12, cathode 14, electrolyte 16) as an electrolyzer cell, the fuel cell being however supplied, with constant flow rates, on its anode with hydrogen (or another fuel such as methane CH4) and on its cathode with oxygen (contained in the sent air) and connected to a load C to deliver the generated electric current.
In the same way as a HTSE electrolyzer, a SOFC comprises a stack of such electrochemical cells separated by interconnection plates for their electric connection and the distribution/collection of gases, which stack may have tightness issues. The cell also comprises a hot area usually submitted to an air sweeping to avoid the accumulation of fuel.
When the stack of the high-temperature fuel cell is tight, in the same way as previously, the seals providing the tightness of the stack in the hot area are fragile and may allow a leaking of:                the fuel (H2, CH4, . . . ) if the leak is located on the anode side, and/or        the depleted air if the leak is located on the cathode side.        
In the same way as previously, in the state of the art, only a detection of the hydrogen leak has been developed, more particularly based on a hydrogen detector or on an explosimeter to analyze the gas sweeping the hot area, as described previously.
In other words, there exists no solution either enabling to detect a leak both on the cathode side and on the anode side of a SOFC in the state of the art. Further, there exists no solution capable of detecting small leaks.